The present invention relates to a system and method for using a multi-contact electrode to stimulate cochlear nerves or other body tissue. More particularly, the invention relates to systems and methods that use multi-channel cochlear nerve stimulation for stimulating individuals who have high stimulation thresholds.
Use of implantable cochlear stimulating devices for restoration of hearing is now a well-accepted modality for treating profound deafness. A cochlear implant system may be fully implantable or partially implantable. In a partially implantable device, there can be two components, an external component containing the battery and an implantable component which contains additional circuitry for processing the stimulation protocol. The implantable component usually consists of a stimulating cochlear lead with an array of multiple electrodes attached to the lead. The stimulating lead with the electrode array is inserted into the cochlea, for instance, into the tympanic chamber (scala tympani). After the electrode array is implanted into the cochlea, the electrodes may be stimulated one at time. In multi-channel systems having independent programmability for each electrode, different stimulus pulse amplitudes and, in some cases, pulsewidths may be delivered at two different electrodes in the same time interval.
The stimulation delivered by an electrode is generally a pulse or a series of pulses. The stimulus pulses are usually biphasic, i.e., the pulses may have a negative first phase and a positive second phase, where the positive second phase is also known as the recharge phase. The negative phase and positive phase are charge balanced to prevent over-accumulation of charges in the tissue adjacent to the stimulating electrode and also to prevent premature corrosion of the stimulating electrode. The negative first phase of the pulse has a time duration. This time duration is commonly referred to as the stimulus “pulse width”. The pulse width as thus defined does not include the duration of the positive second phase.
The stimulation strength or level that just produces stimulation (or capture) of a nerve is termed a “stimulation threshold”. In cochlear stimulation, the stimulation threshold also correlates closely to perception threshold since the firing of only a few ganglion cells (nerve fibers) can be discerned by an individual. To determine stimulation, a stimulus pulse width (pulse duration) is chosen and held constant, for example, 20 microseconds, while amplitude of the pulse is gradually increased. In one method of determining stimulation threshold, the stimulus amplitude is increased until the patient is able to perceive a sound. In an alternative method, the actual neural response (using neural response imaging), or the electrical conduction activity of a cochlear nerve that has been “captured”, may be detected using a recording system when the stimulation threshold has been reached.
The stimulation threshold depends on at least two stimulus parameters: pulse amplitude and pulse duration (or pulse width). The stimulation threshold curve varies inversely between the pulse amplitude and pulse width. Such a threshold curve is also called a strength—duration curve. In accordance with this threshold curve, a larger pulse amplitude may compensate for a reduction in the pulse width to achieve threshold stimulation of a nerve. Alternatively, a larger pulse width can compensate for a smaller pulse amplitude to achieve threshold stimulation.
Normally, stimulation threshold for cochlear applications may be achieved using a stimulus setting of less than about a 50 microsecond pulse width and a current amplitude less than about 1 milliampere. In some poor performing patients, however, it may be necessary to increase the amplitude to the maximum compliance voltage allowed by the stimulator system. These particular patients may be poor performing for a number of reasons. One reason is that disease has caused many nerve cells in the cochlea to die. In addition, the patient may have a peculiar anatomical structure that causes the nerves to be located further away from the stimulating electrodes. As a result, the remaining viable nerves may be dispersed further away from a stimulating electrode and therefore be more difficult to isolate and stimulate.
Sound information is coded in the auditory system in at least two important ways. The first is temporal coding. Temporal information is conveyed as signal information that depends on the rate of firing (frequency) of a nerve fiber or cell. A stimulus may be repeated as a “pulse train” having a specific firing rate or frequency. The variation of stimulus frequency may be translated to frequency of electrical conduction in a specific nerve that is transmitted to the brain, which frequency variation can be perceived as temporal nuances in the sound. Coding of sounds also occurs spatially or spectrally with respect to the arrangement of ganglion cells (nerve fibers) along the cochlea (when the cochlea is viewed as unwound from its coiled state).
The electrode array has a set of electrodes that can be linearly spaced apart along the distal portion of a stimulating lead. As implanted in the cochlea, the electrode array may be placed adjacent to a particular set of cochlear nerve fibers which line the length of the cochlea (modiolus). The nerve fibers are located between the basal (opening) to the apical (tip) of the cochlea and are spatially coded such that certain sound frequencies preferentially stimulate nerve fibers located at the apical ends or the basal end of the cochlea. Thus, by choosing to stimulate through a specific set of electrodes along the cochlea, specific nerve fibers that code for specific sound frequencies can be stimulated. Loudness (intensity) of sound may be coded by recruiting increasing numbers of cochlear nerve fibers. Thus, a just perceptible or threshold sound may occur with stimulation of as few as 3 to 10 ganglion nerve cells, whereas to increase the perceived intensity of the sound, hundreds or even thousands of ganglion nerve cells must be recruited simultaneously.
The typical solution for stimulating a poor performing patient with high stimulation thresholds is to increase the amplitude of the stimulation until the patient reports an adequate loudness percept or until sufficient loudness is determined. A typical problem is that a patient encounters the stimulation output limits of the device before reaching the high stimulation levels necessary to achieve adequate loudness percept. The applied stimulation may be in the form of either voltage stimulation pulses or current stimulation pulses. The maximum available system stimulation level is reached when the voltage stimulation pulses or the current stimulation pulses cannot be increased further because a maximum compliance voltage of the stimulation device has been reached. The maximum compliance voltage is termed the system “compliance voltage.” Increasing the stimulation amplitude, however, can be an inadequate solution because, at such high amplitudes, any “headroom” or extra stimulus amplitude that permits a greater dynamic range of loudness is eliminated. That is, in order to maintain a greater dynamic range of loudness, it is desirable to provide stimulation levels below the system compliance voltage.
In addition, increasing the stimulus amplitude enlarges the current field spread and reduces the spatial specificity or selectivity of an electrode to stimulate sets of nerve fibers because the larger current fields between electrodes tend to overlap or “smear” into other spatial regions containing adjacent sets of nerves during different time intervals. A set of nerves may therefore be stimulated by more than a single electrode, at slightly different times, resulting in a “smearing” effect.
Another way of increasing the stimulus strength is to increase the pulse width. Unfortunately, however, as the pulse width is increased, the temporal information becomes reduced because the wider pulse durations limit the maximum rate of stimulation frequency. Generally, a shorter pulse width can operate at a higher stimulus frequency (pulses per second).
The overall effect therefore in having to increase amplitude or pulse width is to lower the resolution of perceived sound.
It would thus be desirable to have a method of stimulation that can mitigate the resulting loss of information in poor performing patients.
What is needed, therefore, is an improved method of stimulating the auditory nerves, or other tissue being stimulated, that retains temporal and/or spectral information for poor performing patients with high stimulation requirements.